1. Field of the Invention
This invention relates to woven products and methods and apparatus for weaving such products. It particularly relates to woven structural preforms having increased shear strength.
2. Description of Related Developments
Fiber reinforced composites have found wide applicability in the production of structural members. In the aerospace field, composites are used extensively as structural elements for aircraft and spacecraft. The strength characteristics of such structures are influenced strongly by the fiber preform from which the composite is made. Planar single layer fabrics or multilayer three dimensional fabrics having a wide variety of architectures have been used for this purpose. However, known fabric architectures do not provide the ability to resist high shear stress.
Multilayer or three dimensional woven structures which serve as preforms for structural laminates are known. Weaving apparatus for producing multilayer preforms is shown in U.S. Pat. No. 4,019,540 to Holman et al. However, woven structures made in accordance with the teachings of the Holman et al patent do not provide sufficient shear strength or shear stiffness, especially for many aerospace applications. U.S. Pat. No. 4,615,256 to Fukuta et al shows the formation of three dimensional structures by utilizing braiding techniques. However, the process described in this patent is slow and extremely costly and requires equipment of significant size to make preform structures having useful dimensions.
To provide improved shear strength characteristics, triaxial weaving has been proposed, as shown, for example, in U.S. Pat. No. 4,046,173 to Kulczycki, U.S. Pat. No. 4,066,104 to Halton et al, U.S. Pat. No. 4,140,156 to Trost and U.S. Pat. No. 4,438,173 to Trost. The fiber orientations producible by triaxial machines are very limited. Because the process dictates the preform architecture, it is difficult to meet diverse structural demands. Triaxial processes can produce only a single layer fabric and not a multilayer fabric. The fiber architecture resulting from triaxial weaving is very porous and the yarns are heavily crimped. These factors limit the applicability of triaxially woven preforms for aerospace composite applications.